1. Field of the Invention
The present invention relates to an active-matrix display apparatus in which an organic electroluminescence (EL) device is driven using a thin-film transistor (TFT).
2. Description of the Related Art
Organic EL devices are ideal for thin configurations as they emit light and do not require the backlight that is required in liquid crystal displays, and they also do not have restrictions in viewing angle. Thus, the application of organic EL devices is highly expected in the next generation of display devices.
Organic EL display apparatuses can be divided into two types by their structure for selecting and driving the individual organic EL devices; a passive type having a simple matrix structure and an active-matrix type using TFTs. In the active-matrix type, a drive circuit shown in FIG. 1 will be used for each pixel.
In FIG. 1 is shown an organic EL device 3. A drive circuit for one pixel comprises a first TFT 1 for switching, which has a display signal Data1 applied to its drain and which turns on and off by a selection signal SCAN1, a capacitor 2, which is charged by the display signal Data1 that is supplied when the TFT 1 is on and holds a charging voltage Vh1 when the TFT 1 is off, and a second TFT 4, which has its drain connected to a common driving supply COM, its source connected to the anode of the organic EL device 3, and its gate supplied with the holding voltage Vh1 from the capacitor 2 so that the organic EL device 3 is driven with power from the common driving supply COM.
The selection signal SCAN1 then becomes a high level signal during a selected one horizontal scan period (1H) as shown in FIG. 2(a), and the display signal Data1, as shown in FIG. 2(b), is a pulse width modulation signal having a constant pulse amplitude and a pulse width dependent on the emitted luminance to be displayed.
Thus, when the SCAN1 signal goes to a high level and the TFT 1 turns on, the display signal Data1 is supplied to one end of the capacitor 2 via the TFT 1, and the voltage Vh1, which is proportional to the pulse width of the display signal Data1, charges the capacitor 2 as shown in FIG. 2(c). The voltage Vh1 is continuously held at the capacitor 2 during one vertical scan period (1V) even if the SCAN1 goes to a low level and the TFT 1 turns off. Since the voltage Vh1 is being supplied to the gate electrode of the TFT 4, the amount of current supplied to the organic EL device 3 via the TFT 4 is controlled in accordance with the voltage Vh1. As a result thereof, the EL device is controlled to emit light at a luminance proportional to the voltage Vh1. Namely, a gray-scale display is achieved by the pulse width of the display signal Data1.
Generally, the current I versus voltage V characteristic of the EL device has a non-linear relationship as shown in FIG. 3, and the emitted brightness (luminance) B versus voltage V characteristic also has a non-linear relationship as shown in FIG. 3. In particular, the active-matrix device is driven at a relatively low voltage range so that the linearity is worse than that shown in FIG. 3. Thus, xcex3 correction becomes necessary for the image signal to be displayed.
However, for the xcex3-corrected image signal, it is difficult to precisely express gray-scale levels using pulse widths, and as a result, it is difficult to implement a multiple gray-scale display in configurations of the related art.
It is therefore an object of the present invention to solve the above-mentioned shortcomings so as to easily and accurately perform gray-scale display operations.
In order to achieve this object, the present invention is characterized by an electroluminescence display apparatus performing display operations by driving an electroluminescence device having an emissive layer between a pair of electrodes, where the electroluminescence display apparatus comprises: a sampling circuit for sampling an analog video signal at a predetermined period; a capacitor for holding a sampling voltage proportional to the sampled analog video signal that is output from the sampling circuit; a first switch for switching, which is disposed between the sampling circuit and the capacitor and which turns on and off according to a selection signal, for supplying the sampling voltage from the sampling circuit to the capacitor; and a second switch for device driving, which is connected to the electroluminescence device, for the purpose of controlling the light emission at the device by supplying current to the electroluminescence device according to the sampling voltage held at the capacitor.
Another aspect of the present invention is characterized by an electroluminescence display apparatus performing display operations by driving the electroluminescence device having the emissive layer between a pair of electrodes, where the electroluminescence display apparatus comprises: a plurality of display signal lines along rows or columns, and a plurality of selection signal lines disposed so as to intersect with the display signal lines; a sampling circuit for sampling the analog video signal that is input and supplying the analog sampling voltage as a display signal to the corresponding line of the plurality of display signal lines; and a pixel formed near each intersection of the plurality of display signal lines and the plurality of selection signal lines; where the pixel comprises: the electroluminescence device for emitting light according to power supplied from a driving supply; a capacitor for holding the sampling voltage that is supplied as the display signal from the sampling circuit; a first thin-film transistor for switching, which is disposed between the sampling circuit and the capacitor and which turns on and off according to the selection signal that is received as a control signal, for supplying the sampling voltage from the sampling circuit to the capacitor; and the second thin-film transistor for device driving, which is connected to the electroluminescence device, for the purpose of causing the device to emit light by supplying current from the driving supply to the electroluminescence device according to the sampling voltage held at the capacitor.
In still another aspect of the present invention in the above-mentioned electroluminescence display apparatus, the sampling voltage that is sampled at the sampling circuit is a voltage proportional to a gray scale to be displayed.
In this manner, the sampling circuit samples the analog video signal as analog voltage data, and this sampled analog voltage data is held in the capacitor and drives the electroluminescence device. Namely, according to the present invention, analog gray-scale control is possible, and it is possible to accurately and easily perform multiple gray-scale display operations in an active-matrix electroluminescence display apparatus.
Furthermore, in another aspect of the present invention in the above-mentioned electroluminescence display apparatus, the first and second switches in the present invention are thin-film transistors which have their active layer formed from polycrystalline silicon.
If thin-film transistors employing polycrystalline silicon for the active layer are used, high-speed response is possible, and the channel region, source region, and drain region can be formed through self aligning so that the transistors can be formed in a small area. Thus, driving the electroluminescence device using this sort of thin-film transistor easily enables a high-resolution display apparatus to be realized.
Furthermore, in another aspect of the present invention in the above-mentioned electroluminescence display apparatus, the emissive layer of the electroluminescence device includes an organic compound with light emitting function.
In this manner, the electroluminescence device utilizing an organic compound for the emissive layer has a high degree of freedom in the color of emitted light and can realize a high emitted luminance. Thus, using this device in the display apparatus can yield a display apparatus having extremely superior luminescent characteristics.